1. Field of the Invention
The present invention relates to methods and apparatus for accurately measuring the time of arrival of a signal and, more particularly, to enhancing accuracy of time of arrival measurements using a waveform having time-shifted sectors.
2. Description of the Related Art
Accurate determination of signal timing is desirable in a wide variety of communication and navigation applications where precise, reliable signal reception is desired. For example, state-of-the-art position location and communication systems can provide accurate, reliable three-dimensional position determination of a handheld or portable, spread spectrum communication device within milliseconds without interruption of voice or data communications. Among techniques employed to determine the position of a mobile communication device is the reception at the mobile communication device of multiple timing signals respectively transmitted from multiple transmitters at different, known locations. By determining the range to each transmitter from the arrival time of the timing signals, the mobile communication device can compute its position using trilateration. When measuring the range to an object or another device, a precise determination of the signal propagation time between the devices must be made. The signal propagation time can be derived by knowing the transmission and reception times of one or more ranging signals traveling along a direct path between the devices.
The accuracy of the position determined by these systems depends largely on the accuracy with which the receiving devices can determine the time of arrival of the ranging signals traveling along a direct path between the devices. Time-of-Arrival (TOA) measurement accuracy is directly related to the chip rate used in the transmission waveform. Higher TOA accuracy can be achieved with higher chip rates, which increases the transmission bandwidth. Correspondingly higher sampling rates are required to process these higher chip rates in the receiver. Reducing the chip rate to achieve a lower transmit bandwidth requires additional receiver processing to produce TOA accuracy similar to those achieved with the higher chip rate. Further, the TOA waveform length needs to increase as required by the Cramer-ROA bound (CRB) for TOA accuracy. The Cramer-ROA bound (CRB) for TOA accuracy is inversely proportional to bandwidth and the square-root of the operational signal-to-noise ratio. However, increasing the length of the TOA waveform at a lower chip rate requires the stability of the reference oscillators in the radio to improve to minimize frequency error between the transmitter and receiver.
Existing technologies use a delay-lock loop for code tracking or curve fitting techniques to improve the TOA measurement accuracy at lower chip rates. Code tracking with a delay-lock loop requires a feedback loop with either a voltage-controlled-oscillator (VCO) or a numeric-controlled-oscillator (NCO), whose frequency is controlled to properly track the chip timing. To provide better TOA accuracy, the frequency resolution of the VCO/NCO needs to be increased and the loop bandwidth reduced. Nonlinearities associated with the frequency control of an analog VCO approach introduce error in the TOA accuracy. For a digital design using an NCO, improved frequency resolution is achieved by increasing the NCO clock rate, which increases complexity and power consumption. Reduction in the loop bandwidth requirement requires a longer code sequence to obtain the TOA measurement, which impacts the required reference oscillator frequency stability between the transmitter and receiver.
The curve fitting approach determines the TOA measurement by curve fitting the received correlation signal samples to the expected received correlation function. To mitigate multi-path effects, a leading edge curve fitting approach can be used. The number of curve fitting samples collected across the correlation function is determined by the sampling rate. To provide TOA accuracy associated with higher chips using lower chip rates, the sampling rate at the receiver needs to be increased. Increasing the sampling rate enables the correlation function to be mapped out for an improved curve fit. TOA accuracy is improved by the improvement in the curve fitting offered by the higher sampling rate. FIG. 1 shows the received correlation samples for a sample rate four times faster than the chip rate with a −0.1 Tc (Tc=chip period) chip timing error between the transmit and received clocks. Curve fitting is performed by using the eight to ten samples centered about the correlation peak to estimate the correlation function. For leading edge curve fitting, the four to five samples before the correlation peak are used to estimate the leading edge of the correlation function. As shown in FIG. 1, the higher sampling rate provides improved resolution of the correlation function. However, increasing the sampling rate also increases the receiver complexity and its power consumption.
Thus, TOA accuracy improvements using the delay-lock loop for code tracking requires frequency resolution of the VCO/NCO to be increased and the loop bandwidth reduced. Reduction in the loop bandwidth forces a longer code sequence to obtain the TOA measurement, which impacts the required reference oscillator for frequency stability between the transmitter and receiver. These constraints make it difficult to implement the TOA algorithm with short packets in an ad-hoc network system. TOA accuracy improvements using curve fitting require high sampling rates at the receiver for estimation of the correlation function, which increases receiver complexity and power consumption.